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  <p>The domain model, services and repository interfaces. This is the
    central part of the application. The ubiquitous language is used in classes,
    interfaces and method signatures, and every concept in here is familiar to a
    expert in the cargo shipping domain.</p>
  <p>There is no infrastructure or user interface related code here, except
    for things like transactions, validation and security metadata which is
    likely to be relevant to a domain expert ("Either all of foo succeeds or
    none of it does", "In order to do bar you need to be a Supervisor", and so
    on).</p>
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